1. Field of the Invention
The present invention is directed to an improved optical vibration monitor, wherein a light beam is intersected by a light-modifying grid which is mounted so as to oscillate, with respect to the light beam, in response to environmental vibrations.
2. Description of the Related Art
Optical vibration monitors are generally known to be extremely useful, especially in high-electromagnetic environments where traditional vibration sensors, which employ electromagnetic sensing means, would be impractical (i.e., in measuring on-line 120 Hz stator winding end-turn vibrations). Conventional optical vibration sensors employ a grid mounted on the free end of a reed support structure. The grid is generally composed of an opaque plate having a plurality of evenly spaced slits therein through which light may pass. In these arrangements, upon occurrence of environmental vibrations, the reed will vibrate causing the grid to oscillate such that the light normally passing through the spaced slits is periodically interrupted by opaque portions of the opaque plate. A light receiver and suitable evaluation circuitry receive the periodically interrupted light passing through the grid slits, and evaluates the signal, for instance, by comparing the number of light interruptions occurring per cycle of vibration with a threshold value. In this manner, the amplitude of oscillation which the grid experiences, and thus the amplitude of environmental vibration sensed, may be monitored.
However, a problem exists in conventional optical vibration sensors in that the oscillation amplitudes of the grid are dependent on the temperature responsive vibration properties of the reed on which the grid is mounted. Environmental vibrations (i.e., on-line 120 Hz stator winding end-turn vibrations) cause the reed and thus the grid, to oscillate at substantially the same frequency as that of the environmental vibrations (i.e., 120 Hz). The amplitude of the reed vibration in these systems is dependent upon the amplitude of the environmental vibrations, and the difference between the natural frequency of the reed and the frequency of the environmental vibrations. However, an increase in temperature causes the reed's natural frequency to decrease, thereby changing the difference between the natural frequency of the reed and the frequency of the vibration to be sensed. The smaller this difference, the higher will be the vibration amplitude amplification produced by the reed. Thus, if the reed's natural frequency is set to a value above the frequency of the vibration to be sensed, an increase in temperature will result in an increase in amplification factor. In this case, conventional optical vibration sensors will generally produce more light pulses per cycle of grid oscillations at higher temperatures than at lower temperatures for the same degree of environmental vibration. Thus, the accuracy of conventional optical vibration sensors suffers greatly in environments exhibiting temperature variations of any significant degree. By way of example, a conventional optical vibration sensor can employ a reed tuned to 132 Hz to produce an 8.times. amplification of the 120 Hz stator winding end-turn vibrations and can have a temperature dependence of as high as 36 percent over a temperature range of 20.degree.-100.degree. C. (this is about a 0.45 percent per degree temperature dependence).